Biosynthetic production of steviol glycosides rebaudioside J and rebaudioside N
The present disclosure relates to the production of steviol glycosides rebaudioside J and rebaudioside N through the use of rebaudioside A as a substrate and a biosynthetic pathway involving various 1,2 RhaT-rhamnosyltransferases.
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This application is a continuation of International Application No. PCT/US2019/021876, filed on Mar. 12, 2019, which claims priority to U.S. Provisional Application No. 62/641,590, filed on Mar. 12, 2018, and U.S. Provisional Application No. 62/682,260, filed on Jun. 8, 2018, and U.S. Provisional Application No. 62/695,252, filed on Jul. 9, 2018, the contents of each of which are incorporated by reference herein in their entirety.
FIELD OF THE INVENTIONThe present disclosure generally relates to the biosynthesis of steviol glycosides. More specifically, the present disclosure relates to biocatalytic processes for preparing compositions comprising rebaudioside J (“Reb J”) and/or rebaudioside N (“Reb N”), as well as recombinant polypeptides having enzymatic activity useful in the relevant biosynthetic pathways for producing Reb J and/or Reb N.
BACKGROUND OF THE INVENTIONSteviol glycosides are a class of compounds found in the leaves of Stevia rebaudiana plant that can be used as high intensity, low-calorie sweeteners. These naturally occurring steviol glycosides share the same basic diterpene structure (steviol backbone) but differ in the number and type of carbohydrate residues (e.g., glucose, rhamnose, and xylose residues) at the C13 and C19 positions of the steviol backbone. Interestingly, these variations in sugar ‘ornamentation’ of the basic steviol structure often dramatically and unpredictably affect the properties of the resulting steviol glycoside. The properties that are affected can include, without limitation, the overall taste profile, the presence and extent of any off-flavors, crystallization point, “mouth feel”, solubility and perceived sweetness among other differences. Steviol glycosides with known structures include stevioside, rebaudioside A, rebaudioside B, rebaudioside C, rebaudioside D, rebaudioside E, rebaudioside F, rebaudioside M, rebaudioside J, rebaudioside N, and dulcoside A.
On a dry weight basis, stevioside, rebaudioside A, rebaudioside C, and dulcoside A account for approximately 9.1%, 3.8%, 0.6%, and 0.3%, respectively, of the total weight of all steviol glycosides found in wild type stevia leaves. Other steviol glycosides such as Reb J and Reb N are present in significantly lower amounts. Extracts from the Stevia rebaudiana plant are commercially available. In such extracts, stevioside and rebaudioside A typically are the primary components, while the other known steviol glycosides are present as minor or trace components. The actual content level of the various steviol glycosides in any given stevia extract can vary depending on, for example, the climate and soil in which the stevia plants are grown, the conditions under which the stevia leaves are harvested, and the processes used to extract the desired steviol glycosides. To illustrate, the amount of rebaudioside A in commercial preparations can vary from about 20% to more than about 90% by weight of the total steviol glycoside content, while the amount of rebaudioside B, rebaudioside C, and rebaudioside D, respectively, can be about 1-2%, about 7-15%, and about 2% by weight of the total steviol glycoside content. In such extracts, rebaudioside J and rebaudioside N typically account for, individually, less than 0.5% by weight of the total steviol glycoside content.
As natural sweeteners, different steviol glycosides have different degrees of sweetness, mouth feel, and aftertastes. The sweetness of steviol glycosides is significantly higher than that of table sugar (i.e., sucrose). For example, stevioside itself is 100-150 times sweeter than sucrose but has a bitter aftertaste as noted in numerous taste tests, while rebaudiosides A and E are 250-450 times sweeter than sucrose and the aftertaste profile is much better than stevioside. However, these steviol glycosides themselves still retain a noticeable aftertaste. Accordingly, the overall taste profile of any stevia extract is profoundly affected by the relative content of the various steviol glycosides in the extract, which in turn may be affected by the source of the plant, the environmental factors (such as soil content and climate), and the extraction process. In particular, variations of the extraction conditions can lead to inconsistent compositions of the steviol glycosides in the stevia extracts, such that the taste profile varies among different batches of extraction productions. The taste profile of stevia extracts also can be affected by plant-derived or environment-derived contaminants (such as pigments, lipids, proteins, phenolics, and saccharides) that remain in the product after the extraction process. These contaminants typically have off-flavors undesirable for the use of the stevia extract as a sweetener. In addition, the process of isolating individual or specific combinations of steviol glycosides that are not abundant in stevia extracts can be cost- and resource-wise prohibitive.
Further, the extraction process from plants typically employs solid-liquid extraction techniques using solvents such as hexane, chloroform, and ethanol. Solvent extraction is an energy-intensive process, and can lead to problems relating to toxic waste disposal. Thus, new production methods are needed to both reduce the costs of steviol glycoside production as well as to lessen the environmental impact of large scale cultivation and processing.
Accordingly, there is a need in the art for novel preparation methods of steviol glycosides, particularly minor steviol glycosides such as Reb J and Reb N, that can yield products with better and more consistent taste profiles. More preferably, such preparation methods can make use of more abundant steviol glycosides such as Reb A, to reduce the cost of production.
SUMMARY OF THE INVENTIONThe present disclosure encompasses, in some embodiments, a method of preparing Reb J from Reb A as well as the preparation of Reb N from Reb A through the intermediate of Reb J. In some embodiments, the present disclosure provides a method of preparing Reb N from Reb I as well as the preparation of Reb N from Reb A through the intermediate of Reb I.
In one embodiment, the current disclosure provides for the production of steviol glycoside rebaudioside J “Reb J”, or 13-[(2-O-β-D-glucopyranosyl-3-O-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy] ent-kaur-16-en-19-oic acid-[(2-O-α-L-rhamnopyranosyl-β-D-glucopyranosyl) ester], by various 1,2-rhamnosyltransferase enzymes described herein from Reb A.
In another embodiment, the current disclosure provides for the production of steviol glycoside rebaudioside N “Reb N”, or 13-[(2-O-β-D-glucopyranosyl-3-O-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy] ent-kaur-16-en-19-oic acid-[(2-O-α-L-rhamnopyranosyl-3-O-β-D-glucopyranosyl-β-D-glucopyranosyl) ester] by various UDP-glycosyltransferases described herein from Reb J.
The current method provides an approach for the synthesis of specific steviol glycosides using certain specific synthetic pathways.
In terms of product/commercial utility there are several dozen products containing steviol glycosides on the market in the United States and can be used in everything from foods, beverages, and dietary supplements to analgesics and pest repellents. Products containing steviol glycosides can be liquids, granular formulations, gels or aerosols.
Provided herein, inter alia, are biosynthetic methods of preparing a rebaudioside, such as rebaudioside N, the methods comprising reacting a steviol glycoside composition with a rhamnose donor moiety in the presence of a first recombinant polypeptide having 1,2-rhamnosytransferase activity; wherein the first recombinant polypeptide comprises an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) sequence identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19.
In some embodiments, the first recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 3.
In some embodiments, the first recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 9.
In some embodiments, the first recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 19.
In some embodiments, biosynthetic methods provided herein comprise expressing the first recombinant polypeptide in a transformed cellular system. In some embodiments, the transformed cellular system is selected from the group consisting of a yeast, a non-steviol glycoside producing plant, an alga, a fungus, and a bacterium. In some embodiments, the bacterium or yeast is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; and Clostridium.
In some embodiments, biosynthetic methods provided herein comprise a reacting step that is performed in the transformed cellular system. In other embodiments, the reacting step can be performed in vitro. In some embodiments, biosynthetic methods comprise isolating the first recombinant polypeptide from the transformed cellular system and the reacting step can be performed in vitro.
In some embodiments, the rhamnose donor is rhamnose. In some embodiments, the rhamnose donor is L-rhamnose. In some embodiments, the rhamnose donor moiety is UDP-L-rhamnose.
In some embodiments, the steviol glycoside composition comprises rebaudioside A and the reacting step leads to the production of rebaudioside J.
In some embodiments, biosynthetic methods provided herein further comprise reacting the rebaudioside J with a glucose donor moiety in the presence of a second recombinant polypeptide having glucosytransferase activity. In some embodiments, the glucose donor moiety is generated in situ.
In some embodiments, the second recombinant polypeptide has both glucosyltransferase activity and sucrose synthase activity. In some embodiments, the second recombinant polypeptide comprises an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) sequence identity to SEQ ID NO: 7, SEQ ID NO: 11, SEQ ID NO: 13, or SEQ ID NO: 15.
In some embodiments, the second recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 7.
In some embodiments, the second recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 11.
In some embodiments, the second recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 13.
In some embodiments, the second recombinant polypeptide comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 15.
In some embodiments, biosynthetic methods provided herein further comprise reacting the rebaudioside J with a glucose donor moiety in the presence of a third recombinant polypeptide having sucrose synthase activity.
In some embodiments, the steviol glycoside composition can include rebaudioside I. In some embodiments, the rebaudioside I can be prepared by reacting a steviol glycoside composition comprising rebaudioside A with a glucose donor moiety in the presence of a second recombinant polypeptide having glucosyltransferase activity.
In some embodiments, biosynthetic methods provided herein further comprise reacting the steviol glycoside composition comprising rebaudioside A with a glucose donor moiety in the presence of a third recombinant polypeptide having sucrose synthase activity.
Also provided herein, inter alia, are biosynthetic methods of preparing a rebaudioside, such as rebaudioside J, the methods comprising reacting a steviol glycoside composition comprising rebaudioside A with a rhamnose donor moiety in the presence of a recombinant polypeptide having 1,2-rhamnosytransferase activity; wherein said recombinant polypeptide comprises an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) sequence identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19.
In some embodiments, the recombinant polypeptide having 1,2-rhamnosytransferase activity comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 3.
In some embodiments, the recombinant polypeptide having 1,2-rhamnosytransferase activity comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 9.
In some embodiments, the recombinant polypeptide having 1,2-rhamnosytransferase activity comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 19.
Aspects of the present disclosure also provide a rebaudioside obtainable by or produced by any biosynthetic method described herein, including any of the above-mentioned embodiments.
Aspects of the present disclosure also provide a nucleic acid encoding a polypeptide as described herein. In some embodiments, the nucleic acid comprises a sequence encoding a polypeptide comprising an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the nucleic acid comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 4, SEQ ID NO: 10, or SEQ ID NO: 20. In some embodiments, the nucleic acid comprises the sequence of SEQ ID NO: 4. In some embodiments, the nucleic acid comprises the sequence of SEQ ID NO: 10. In some embodiments, the nucleic acid comprises the sequence of SEQ ID NO: 20. In some embodiments, the nucleic acid is a plasmid or other vector.
Aspects of the present disclosure also provide a cell comprising a nucleic acid described herein, including any of the above-mentioned embodiments.
Other aspects of the present disclosure provide a composition comprising at least one polypeptide comprising an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the composition comprises at least one polypeptide comprising the sequence of SEQ ID NO: 3. In some embodiments, the composition comprises at least one polypeptide comprising the sequence of SEQ ID NO: 9. In some embodiments, the composition comprises at least one polypeptide comprising the sequence of SEQ ID NO: 19. In some embodiments, the composition is an in vitro reaction mixture, e.g., comprising a rhamnose donor moiety as described herein and a steviol glycoside composition as described herein.
Aspects of the present disclosure provide a cell comprising at least one polypeptide comprising an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or 100% identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the cell comprises at least one polypeptide comprising the sequence of SEQ ID NO: 3. In some embodiments, the cell comprises at least one polypeptide comprising the sequence of SEQ ID NO: 9. In some embodiments, the cell comprises at least one polypeptide comprising the sequence of SEQ ID NO: 19. In some embodiments, the cell is a yeast cell, a non-steviol glycoside producing plant cell, an algal cell, a fungal cell, or a bacterial cell. In some embodiments, the bacterium or yeast cell is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; and Clostridium. In some embodiments, the cell further comprises a second polypeptide having glucosytransferase activity or glucosyltransferase activity and sucrose synthase activity as described herein, such as a second polypeptide comprising an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) sequence identity to SEQ ID NO: 7, SEQ ID NO: 11, SEQ ID NO: 13, or SEQ ID NO: 15.
As for the cellular system in the embodiment, it can be selected from the group consisting of one or more bacteria, one or more yeasts, and a combination thereof, or any cellular system that would allow the genetic transformation with the selected genes and thereafter the biosynthetic production of the desired steviol glycosides. In a most preferred microbial system, E. coli are used to produce the desired steviol glycoside compounds.
In some embodiments, the disclosure provides a mutant of EU11 enzyme comprising the amino acid sequence of SEQ ID NO: 3 and identified as EUCP1. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 3.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to EUCP1 and comprising SEQ ID NO: 4. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 4.
In some embodiments, the disclosure provides a mutant of the EU11 enzyme comprising the amino acid sequence of SEQ ID NO: 1. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 1.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to EU11 and comprising SEQ ID NO: 2. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 2.
In some embodiments, the disclosure provides an enzyme referred herein as UGT2E-B comprising the amino acid sequence of SEQ ID NO: 9. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 9.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to UGT2E-B and comprising SEQ ID NO: 10. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 10.
In some embodiments, the disclosure provides an enzyme referred herein as NX114 comprising the amino acid sequence of SEQ ID NO: 19. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 19.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to NX114 and comprising SEQ ID NO: 20. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 20.
In some embodiments, the disclosure provides a microbial host cell comprising a vector capable of producing one or more enzymes described herein. In certain embodiments, the enzyme can be selected from the group consisting of EUCP1 [SEQ ID. NO. 3], UGT2E-B [SEQ ID. No. 9], and NX114 [SEQ ID NO. 19]. In some embodiments, the enzyme can be selected from the group consisting of CP1 [SEQ ID NO. 11] and CP2 [SEQ ID NO. 13]. In some embodiments, the enzyme can be a fusion enzyme referred herein as GS [SEQ ID No. 15]. In some embodiments, the host cell is selected from the group consisting of a bacterium, a yeast, a filamentous fungus, a cyanobacteria alga and a plant cell. In some embodiments, the host cell is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; Corynebacterium; Clostridium (e.g., Clostridium acetobutylicum). In some embodiments, the host cell is a cell isolated from plants selected from the group consisting of soybean; rapeseed; sunflower; cotton; corn; tobacco; alfalfa; wheat; barley; oats; sorghum; rice; broccoli; cauliflower; cabbage; parsnips; melons; carrots; celery; parsley; tomatoes; potatoes; strawberries; peanuts; grapes; grass seed crops; sugar beets; sugar cane; beans; peas; rye; flax; hardwood trees; softwood trees; forage grasses; Arabidopsis thaliana; rice (Oryza sativa); Hordeum yulgare; switchgrass (Panicum vigratum); Brachypodium spp.; Brassica spp.; and Crambe abyssinica.
In some embodiments, the disclosure provides a method of producing rebaudioside J, the method comprising incubating a substrate with a recombinant polypeptide comprising an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the recombinant polypeptide is a 1,2 rhamnosyltranferase having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the substrate is selected from the group consisting of rubusoside, stevioside or rebaudioside A and combinations thereof.
In some embodiments, the disclosure provides a sweetener comprising Reb J produced by any of the embodiments of the above-mentioned method.
In some embodiments, the disclosure provides a method of producing rebaudioside N, the method comprising incubating a substrate with a first recombinant polypeptide comprising an amino acid sequence having at least 80% (e.g., at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the substrate is selected from the group consisting of Reb J, Reb I, Reb A, stevioside or rubusoside and combinations thereof. In some embodiments, the substrate comprises Reb J. In other embodiments, the substrate comprises Reb I. In some embodiments, the method further comprises incubating a second recombinant polypeptide having glucosyltransferase activity and optionally a recombinant sucrose synthase, with the substrate and the first recombinant polypeptide.
In some embodiments, the disclosure provides a sweetener comprising Reb N produced by any of the embodiments of the above-mentioned method.
In some embodiments, the disclosure provides a method for synthesizing rebaudioside N from rebaudioside J, the method comprising: preparing a reaction mixture comprising rebaudioside J, a glucose donor moiety selected from the group consisting of sucrose, uridine diphosphate (UDP) and uridine diphosphate glucose (UDP-glucose), and a UGT enzyme, incubating the reaction mixture for a sufficient time to produce rebaudioside N, wherein a glucose is covalently coupled to the rebaudioside J to produce rebaudioside N. In various embodiments, the UGT enzyme can be a polypeptide comprising an amino acid sequence having at least 80% sequence identity to UGT76G1 [SEQ ID No. 7], CP1 [SEQ ID No. 11], CP2 [SEQ ID No. 13], or GS [SEQ ID No. 15]. In some embodiments, the method further comprises adding a sucrose synthase to the reaction mixture. In some embodiments, the sucrose synthase is selected from the group consisting of an Arabidopsis sucrose synthase 1, an Arabidopsis sucrose synthase 3 and a Vigna radiate sucrose synthase. In some embodiments, the sucrose synthase is an Arabidopsis thaliana sucrose synthase 1 (SEQ ID NO: 17).
In some embodiments, said Reb N is greater than 70% (e.g., greater than 80%, greater than 85%, greater than 90%, greater than 95%, greater than 96%, greater than 97%, greater than 98%, or greater than 99%) pure.
In some embodiments, the disclosure provides a host cell comprising a vector capable of producing an enzyme wherein the amino acid sequence corresponds to SEQ ID NO: 3, SEQ ID NO: 9, or SEQ ID NO: 19. In some embodiments, the host cell is selected from the group consisting of a bacterium, a yeast, a filamentous fungus, a cyanobacteria alga and a plant cell. In some embodiments, the host cell is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; Corynebacterium; Clostridium. In some embodiments, the host cell is a cell isolated from plants selected from the group consisting of soybean; rapeseed; sunflower; cotton; corn; tobacco; alfalfa; wheat; barley; oats; sorghum; rice; broccoli; cauliflower; cabbage; parsnips; melons; carrots; celery; parsley; tomatoes; potatoes; strawberries; peanuts; grapes; grass seed crops; sugar beets; sugar cane; beans; peas; rye; flax; hardwood trees; softwood trees; forage grasses; Arabidopsis thaliana; rice (Oryza sativa); Hordeum yulgare; switchgrass (Panicum vigratum); Brachypodium spp.; Brassica spp.; and Crambe abyssinica.
In some embodiments, the disclosure provides a beverage product comprising: up to about 125 ppm Rebaudioside N; and at least one non-nutritive sweetener selected from the group consisting of Reb J; Reb W; Reb V; Reb D4, Reb E, and Reb M, and combinations thereof, wherein the at least one non-nutritive sweetener is present in a concentration from about 30 ppm to about 600 ppm. In some embodiments, the at least one non-nutritive sweetener is selected from the group consisting of Reb J; Reb W; Reb V; Reb D4, Reb E, and Reb M, and combinations thereof, and wherein the Reb N and the at least one non-nutritive sweetener are present in a weight ratio of about 1:5.
In some embodiments, the disclosure provides a GS fusion enzyme comprising a first domain having an amino acid sequence at least 90% identical to UGT76G1 and a second domain having an amino acid sequence at least 90% identical to AtSUS1. The GS fusion enzyme can have an amino acid sequence comprising SEQ ID NO: 15. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 15.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to GS and comprising SEQ ID NO: 16. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 16.
In some embodiments, the disclosure provides a UDP-glycosyltransferase comprising the UGT2E-B enzyme and having the amino acid sequence of SEQ ID NO: 9. In some embodiments, the disclosure provides a recombinant polypeptide comprising an amino acid sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 9.
In some embodiments, the disclosure provides a DNA molecule having a sequence corresponding to UGT2E-B and comprising SEQ ID NO: 10. In some embodiments, the disclosure provides a nucleic acid molecule having a sequence having at least 90% (e.g., at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100%) identity to SEQ ID NO: 10.
In some embodiments, the disclosure provides a microbial host cell comprising a vector capable of producing the UGT2E-B enzyme. In some embodiments, the host cell is selected from the group consisting of a bacterium, a yeast, a filamentous fungus, a cyanobacteria alga and a plant cell. In some embodiments, the host cell is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; Corynebacterium; Clostridium. In some embodiments, the host cell is a cell isolated from plants selected from the group consisting of soybean; rapeseed; sunflower; cotton; corn; tobacco; alfalfa; wheat; barley; oats; sorghum; rice; broccoli; cauliflower; cabbage; parsnips; melons; carrots; celery; parsley; tomatoes; potatoes; strawberries; peanuts; grapes; grass seed crops; sugar beets; sugar cane; beans; peas; rye; flax; hardwood trees; softwood trees; forage grasses; Arabidopsis thaliana; rice (Oryza sativa); Hordeum yulgare; switchgrass (Panicum vigratum); Brachypodium spp.; Brassica spp.; and, Crambe abyssinica.
In some embodiments, the disclosure provides a method of producing rebaudioside N, the method comprising incubating a substrate with a recombinant polypeptide comprising an amino acid sequence having at least 80% identity to SEQ ID NO: 11 or SEQ ID NO: 13 or SEQ ID NO: 15. In some embodiments, the substrate is selected from the group consisting of Reb J, Reb A, stevioside or rubusoside and combinations thereof. In some embodiments, the method further comprises incubating a recombinant sucrose synthase with the substrate and the recombinant polypeptide.
In some embodiments, the disclosure provides a host cell comprising a vector capable of producing the CP1 and CP2 enzymes wherein the amino acid sequence corresponds to SEQ ID NOs: 11 and 13, respectively. In some embodiments, the host cell is selected from the group consisting of a bacterium, a yeast, a filamentous fungus, a cyanobacteria alga and a plant cell. In some embodiments, the host cell is selected from the group consisting of Escherichia; Salmonella; Bacillus; Acinetobacter; Streptomyces; Corynebacterium; Methylosinus; Methylomonas; Rhodococcus; Pseudomonas; Rhodobacter; Synechocystis; Saccharomyces; Zygosaccharomyces; Kluyveromyces; Candida; Hansenula; Debaryomyces; Mucor; Pichia; Torulopsis; Aspergillus; Arthrobotlys; Brevibacteria; Microbacterium; Arthrobacter; Citrobacter; Klebsiella; Pantoea; Corynebacterium; Clostridium. In some embodiments, the host cell is a cell isolated from plants selected from the group consisting of soybean; rapeseed; sunflower; cotton; corn; tobacco; alfalfa; wheat; barley; oats; sorghum; rice; broccoli; cauliflower; cabbage; parsnips; melons; carrots; celery; parsley; tomatoes; potatoes; strawberries; peanuts; grapes; grass seed crops; sugar beets; sugar cane; beans; peas; rye; flax; hardwood trees; softwood trees; forage grasses; Arabidopsis thaliana; rice (Oryza sativa); Hordeum yulgare; switchgrass (Panicum vigratum); Brachypodium spp.; Brassica spp.; and, Crambe abyssinica.
In some embodiments, the disclosure provides a mutant enzyme comprising the amino acid sequence of SEQ ID NO: 11 and identified as CP1.
In some embodiments, the disclosure provides a mutant enzyme comprising the amino acid sequence of SEQ ID NO: 13 and identified as CP2.
While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawing and will herein be described in detail. It should be understood, however, that the drawings and detailed description presented herein are not intended to limit the disclosure to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
Other features and advantages of this invention will become apparent in the following detailed description of preferred embodiments of this invention, taken with reference to the accompanying drawings.
Steviol Glycosides are a class of chemical compounds responsible for the sweet taste of the leaves of the South American plant Stevia rebaudiana (Asteraceae), and can be used as sweeteners in food, feed and beverages.
DefinitionsCellular system is any cells that provide for the expression of ectopic proteins. It included bacteria, yeast, plant cells and animal cells. It includes both prokaryotic and eukaryotic cells. It also includes the in vitro expression of proteins based on cellular components, such as ribosomes.
Coding sequence is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is used without limitation to refer to a DNA sequence that encodes for a specific amino acid sequence.
Growing the Cellular System. Growing includes providing an appropriate medium that would allow cells to multiply and divide. It also includes providing resources so that cells or cellular components can translate and make recombinant proteins.
Protein Expression. Protein production can occur after gene expression. It consists of the stages after DNA has been transcribed to messenger RNA (mRNA). The mRNA is then translated into polypeptide chains, which are ultimately folded into proteins. DNA is present in the cells through transfection—a process of deliberately introducing nucleic acids into cells. The term is often used for non-viral methods in eukaryotic cells. It may also refer to other methods and cell types, although other terms are preferred: “transformation” is more often used to describe non-viral DNA transfer in bacteria, non-animal eukaryotic cells, including plant cells. In animal cells, transfection is the preferred term as transformation is also used to refer to progression to a cancerous state (carcinogenesis) in these cells. Transduction is often used to describe virus-mediated DNA transfer. Transformation, transduction, and viral infection are included under the definition of transfection for this application.
Yeast. According to the current disclosure a yeast as claimed herein are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. Yeasts are unicellular organisms which evolved from multicellular ancestors but with some species useful for the current disclosure being those that have the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudo hyphae or false hyphae.
UGT Names. The names of the UGT enzymes used in the present disclosure are consistent with the nomenclature system adopted by the UGT Nomenclature Committee (Mackenzie et al., “The UDP glycosyltransferase gene super family: recommended nomenclature updated based on evolutionary divergence,” P
As used herein, the singular forms “a, an” and “the” include plural references unless the content clearly dictates otherwise.
To the extent that the term “include,” “have,” or the like is used in the description or the claims, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim.
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.
The term “complementary” is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is used without limitation to describe the relationship between nucleotide bases that are capable to hybridizing to one another. For example, with respect to DNA, adenosine is complementary to thymine and cytosine is complementary to guanine. Accordingly, the subjection technology also includes isolated nucleic acid fragments that are complementary to the complete sequences as reported in the accompanying Sequence Listing as well as those substantially similar nucleic acid sequences.
The terms “nucleic acid” and “nucleotide” are to be given their respective ordinary and customary meanings to a person of ordinary skill in the art and are used without limitation to refer to deoxyribonucleotides or ribonucleotides and polymers thereof in either single- or double-stranded form. Unless specifically limited, the term encompasses nucleic acids containing known analogues of natural nucleotides that have similar binding properties as the reference nucleic acid and are metabolized in a manner similar to naturally-occurring nucleotides. Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified or degenerate variants thereof (e.g., degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated.
The term “isolated” is to be given its ordinary and customary meaning to a person of ordinary skill in the art, and when used in the context of an isolated nucleic acid or an isolated polypeptide, is used without limitation to refer to a nucleic acid or polypeptide that, by the hand of man, exists apart from its native environment and is therefore not a product of nature. An isolated nucleic acid or polypeptide can exist in a purified form or can exist in a non-native environment such as, for example, in a transgenic host cell.
The terms “incubating” and “incubation” as used herein means a process of mixing two or more chemical or biological entities (such as a chemical compound and an enzyme) and allowing them to interact under conditions favorable for producing a steviol glycoside composition.
The term “degenerate variant” refers to a nucleic acid sequence having a residue sequence that differs from a reference nucleic acid sequence by one or more degenerate codon substitutions. Degenerate codon substitutions can be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed base and/or deoxy inosine residues. A nucleic acid sequence and all of its degenerate variants will express the same amino acid or polypeptide.
The terms “polypeptide,” “protein,’ and “peptide” are to be given their respective ordinary’ and customary meanings to a person of ordinary skill in the art; the three terms are sometimes used interchangeably and are used without limitation to refer to a polymer of amino acids, or amino acid analogs, regardless of its size or function. Although “protein” is often used in reference to relatively large polypeptides, and “peptide” is often used in reference to small polypeptides, usage of these terms in the art overlaps and varies. The term ‘polypeptide” as used herein refers to peptides, polypeptides, and proteins, unless otherwise noted. The terms “protein,” “polypeptide,” and “peptide” are used interchangeably herein when referring to a polynucleotide product. Thus, exemplary polypeptides include polynucleotide products, naturally occurring proteins, homologs, orthologs, paralogs, fragments and other equivalents, variants, and analogs of the foregoing.
The terms “polypeptide fragment” and “fragment,” when used in reference to a reference polypeptide, are to be given their ordinary and customary meanings to a person of ordinary skill in the art and are used without limitation to refer to a polypeptide in which amino acid residues are deleted as compared to the reference polypeptide itself, but where the remaining amino acid sequence is usually identical to the corresponding positions in the reference polypeptide. Such deletions can occur at the amino-terminus or carboxy-terminus of the reference polypeptide, or alternatively both.
The term “functional fragment” of a polypeptide or protein refers to a peptide fragment that is a portion of the full-length polypeptide or protein, and has substantially the same biological activity, or carries out substantially the same function as the full-length polypeptide or protein (e.g., carrying out the same enzymatic reaction).
The terms “variant polypeptide,” “modified amino acid sequence” or “modified polypeptide,” which are used interchangeably, refer to an amino acid sequence that is different from the reference polypeptide by one or more amino acids, e.g., by one or more amino acid substitutions, deletions, and/or additions. In an aspect, a variant is a “functional variant” which retains some or all of the ability of the reference polypeptide.
The term “functional variant” further includes conservatively substituted variants. The term “conservatively substituted variant” refers to a peptide having an amino acid sequence that differs from a reference peptide by one or more conservative amino acid substitutions and maintains some or all of the activity of the reference peptide. A “conservative amino acid substitution” is a substitution of an amino acid residue with a functionally similar residue. Examples of conservative substitutions include the substitution of one non-polar (hydrophobic) residue such as isoleucine, valine, leucine or methionine for another; the substitution of one charged or polar (hydrophilic) residue for another such as between arginine and lysine, between glutamine and asparagine, between threonine and serine; the substitution of one basic residue such as lysine or arginine for another; or the substitution of one acidic residue, such as aspartic acid or glutamic acid for another; or the substitution of one aromatic residue, such as phenylalanine, tyrosine, or tryptophan for another. Such substitutions are expected to have little or no effect on the apparent molecular weight or isoelectric point of the protein or polypeptide. The phrase “conservatively substituted variant” also includes peptides wherein a residue is replaced with a chemically-derivatized residue, provided that the resulting peptide maintains some or all of the activity of the reference peptide as described herein.
The term “variant,” in connection with the polypeptides of the subject technology, further includes a functionally active polypeptide having an amino acid sequence at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, and even 100% identical to the amino acid sequence of a reference polypeptide.
The term “homologous” in all its grammatical forms and spelling variations refers to the relationship between polynucleotides or polypeptides that possess a “common evolutionary origin,” including polynucleotides or polypeptides from super families and homologous polynucleotides or proteins from different species (Reeck et al., C
“Suitable regulatory sequences” is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is used without limitation to refer to nucleotide sequences located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, and polyadenylation recognition sequences.
“Promoter” is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is used without limitation to refer to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3′ to a promoter sequence. Promoters may be derived in their entirety from a native gene or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental conditions. Promoters, which cause a gene to be expressed in most cell types at most times, are commonly referred to as “constitutive promoters”. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity.
The term “operably linked” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it can affect the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or anti sense orientation.
The term “expression” as used herein, is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is used without limitation to refer to the transcription and stable accumulation of sense (mRNA) or anti sense RNA derived from the nucleic acid fragment of the subject technology. “Over-expression” refers to the production of a gene product in transgenic or recombinant organisms that exceeds levels of production in normal or non-transformed organisms.
“Transformation” is to be given its ordinary and customary meaning to a person of reasonable skill in the craft and is used without limitation to refer to the transfer of a polynucleotide into a target cell. The transferred polynucleotide can be incorporated into the genome or chromosomal DNA of a target cell, resulting in genetically stable inheritance, or it can replicate independent of the host chromosomal. Host organisms containing the transformed nucleic acid fragments are referred to as “transgenic” or “transformed”.
The terms “transformed,” “transgenic,” and “recombinant,” when used herein in connection with host cells, are to be given their respective ordinary and customary meanings to a person of ordinary skill in the art and are used without limitation to refer to a cell of a host organism, such as a plant or microbial cell, into which a heterologous nucleic acid molecule has been introduced. The nucleic acid molecule can be stably integrated into the genome of the host cell, or the nucleic acid molecule can be present as an extrachromosomal molecule. Such an extrachromosomal molecule can be auto-replicating. Transformed cells, tissues, or subjects are understood to encompass not only the end product of a transformation process, but also transgenic progeny thereof.
The terms “recombinant,” “heterologous,” and “exogenous,” when used herein in connection with polynucleotides, are to be given their ordinary and customary meanings to a person of ordinary skill in the art and are used without limitation to refer to a polynucleotide (e.g., a DNA sequence or a gene) that originates from a source foreign to the particular host cell or, if from the same source, is modified from its original form. Thus, a heterologous gene in a host cell includes a gene that is endogenous to the particular host cell but has been modified through, for example, the use of site-directed mutagenesis or other recombinant techniques. The terms also include non-naturally occurring multiple copies of a naturally occurring DNA sequence. Thus, the terms refer to a DNA segment that is foreign or heterologous to the cell, or homologous to the cell but in a position or form within the host cell in which the element is not ordinarily found.
Similarly, the terms “recombinant,” “heterologous,” and “exogenous,” when used herein in connection with a polypeptide or amino acid sequence, means a polypeptide or amino acid sequence that originates from a source foreign to the particular host cell or, if from the same source, is modified from its original form. Thus, recombinant DNA segments can be expressed in a host cell to produce a recombinant polypeptide.
The terms “plasmid,” “vector,” and “cassette” are to be given their respective ordinary and customary meanings to a person of ordinary skill in the art and are used without limitation to refer to an extra chromosomal element often carrying genes which are not part of the central metabolism of the cell, and usually in the form of circular double-stranded DNA molecules. Such elements may be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear or circular, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence into a cell. “Transformation cassette” refers to a specific vector containing a foreign gene and having elements in addition to the foreign gene that facilitate transformation of a particular host cell. “Expression cassette” refers to a specific vector containing a foreign gene and having elements in addition to the foreign gene that allow for enhanced expression of that gene in a foreign host.
The present disclosure relates, in some embodiments, to the production of a steviol glycoside of interest, Reb N, from Reb A using at least one novel UDP-rhamnosyltransferases (RhaT) described herein, which can transfer a rhamnose moiety from UDP-L-rhamnose to a steviol glycoside acceptor in a rhamnosylation reaction. Referring to
Synthetic Biology
Standard recombinant DNA and molecular cloning techniques used here are well known in the art and are described, for example, by Sambrook, J., Fritsch, E. F. and Maniatis, T. M
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure belongs. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the present disclosure, the preferred materials and methods are described below.
The disclosure will be more fully understood upon consideration of the following non-limiting Examples. It should be understood that these Examples, while indicating preferred embodiments of the subject technology, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of the subject technology, and without departing from the spirit and scope thereof, can make various changes and modifications of the subject technology to adapt it to various uses and conditions.
Glycosylation is often considered a ubiquitous reaction controlling the bioactivity and storage of plant natural products. Glycosylation of small molecules is catalyzed by a superfamily of transferases in most plant species that have been studied to date. These glycosyltransferases (GTs) have been classified into over 60 families. Of these, the family of GT enzymes, also known as the UDP glycosyltransferases (UGTs) and UDP-rhamnosyltransferase, transfers sugar moieties to specific acceptor molecules. These are the molecules that transfer such sugar moieties in the steviol glycosides to help create various rebaudiosides. Each of these enzymes have their own activity profile and preferred structure locations where they transfer their activated sugar moieties.
Production Systems
Expression of proteins in prokaryotes is most often carried out in a bacterial host cell with vectors containing constitutive or inducible promoters directing the expression of either fusion or non-fusion proteins. Fusion vectors add a number of amino acids to a protein encoded therein, usually to the amino terminus of the recombinant protein. Such fusion vectors typically serve three purposes: 1) to increase expression of recombinant protein; 2) to increase the solubility of the recombinant protein; and, 3) to aid in the purification of the recombinant protein by acting as a ligand in affinity purification. Often, a proteolytic cleavage site is introduced at the junction of the fusion moiety and the recombinant protein to enable separation of the recombinant protein from the fusion moiety subsequent to purification of the fusion protein. Such vectors are within the scope of the present disclosure.
In an embodiment, the expression vector includes those genetic elements for expression of the recombinant polypeptide in bacterial cells. The elements for transcription and translation in the bacterial cell can include a promoter, a coding region for the protein complex, and a transcriptional terminator.
A person of ordinary skill in the art will be aware of the molecular biology techniques available for the preparation of expression vectors. The polynucleotide used for incorporation into the expression vector of the subject technology, as described above, can be prepared by routine techniques such as polymerase chain reaction (PCR).
Several molecular biology techniques have been developed to operably link DNA to vectors via complementary cohesive termini. In one embodiment, complementary homopolymer tracts can be added to the nucleic acid molecule to be inserted into the vector DNA. The vector and nucleic acid molecule are then joined by hydrogen bonding between the complementary homopolymeric tails to form recombinant DNA molecules.
In an alternative embodiment, synthetic linkers containing one or more restriction sites provide are used to operably link the polynucleotide of the subject technology to the expression vector. In an embodiment, the polynucleotide is generated by restriction endonuclease digestion. In an embodiment, the nucleic acid molecule is treated with bacteriophage T4 DNA polymerase or E. coli DNA polymerase I, enzymes that remove protruding, 3′-single-stranded termini with their 3′-5′-exonucleolytic activities and fill in recessed 3′-ends with their polymerizing activities, thereby generating blunt ended DNA segments. The blunt-ended segments are then incubated with a large molar excess of linker molecules in the presence of an enzyme that can catalyze the ligation of blunt-ended DNA molecules, such as bacteriophage T4 DNA ligase. Thus, the product of the reaction is a polynucleotide carrying polymeric linker sequences at its ends. These polynucleotides are then cleaved with the appropriate restriction enzyme and ligated to an expression vector that has been cleaved with an enzyme that produces termini compatible with those of the polynucleotide.
Alternatively, a vector having ligation-independent cloning (LIC) sites can be employed. The required PCR amplified polynucleotide can then be cloned into the LIC vector without restriction digest or ligation (Aslanidis and de Jong, N
In an embodiment, to isolate and/or modify the polynucleotide of interest for insertion into the chosen plasmid, it is suitable to use PCR. Appropriate primers for use in PCR preparation of the sequence can be designed to isolate the required coding region of the nucleic acid molecule, add restriction endonuclease or LIC sites, place the coding region in the desired reading frame.
In an embodiment, a polynucleotide for incorporation into an expression vector of the subject technology is prepared using PCR using appropriate oligonucleotide primers. The coding region is amplified, whilst the primers themselves become incorporated into the amplified sequence product. In an embodiment, the amplification primers contain restriction endonuclease recognition sites, which allow the amplified sequence product to be cloned into an appropriate vector.
The expression vectors can be introduced into plant or microbial host cells by conventional transformation or transfection techniques. Transformation of appropriate cells with an expression vector of the subject technology is accomplished by methods known in the art and typically depends on both the type of vector and cell. Suitable techniques include calcium phosphate or calcium chloride co-precipitation, DEAE-dextran mediated transfection, lipofection, chemoporation or electroporation.
Successfully transformed cells, that is, those cells containing the expression vector, can be identified by techniques well known in the art. For example, cells transfected with an expression vector of the subject technology can be cultured to produce polypeptides described herein. Cells can be examined for the presence of the expression vector DNA by techniques well known in the art.
The host cells can contain a single copy of the expression vector described previously, or alternatively, multiple copies of the expression vector,
In some embodiments, the transformed cell is an animal cell, an insect cell, a plant cell, an algal cell, a fungal cell, or a yeast cell. In some embodiments, the cell is a plant cell selected from the group consisting of: canola plant cell, a rapeseed plant cell, a palm plant cell, a sunflower plant cell, a cotton plant cell, a corn plant cell, a peanut plant cell, a flax plant cell, a sesame plant cell, a soybean plant cell, and a petunia plant cell.
Microbial host cell expression systems and expression vectors containing regulatory sequences that direct high-level expression of foreign proteins are well known to those skilled in the art. Any of these could be used to construct vectors for expression of the recombinant polypeptide of the subjection technology in a microbial host cell. These vectors could then be introduced into appropriate microorganisms via transformation to allow for high level expression of the recombinant polypeptide of the subject technology.
Vectors or cassettes useful for the transformation of suitable microbial host cells are well known in the art. Typically, the vector or cassette contains sequences directing transcription and translation of the relevant polynucleotide, a selectable marker, and sequences allowing autonomous replication or chromosomal integration. Suitable vectors comprise a region 5′ of the polynucleotide which harbors transcriptional initiation controls and a region 3′ of the DNA fragment which controls transcriptional termination. It is preferred for both control regions to be derived from genes homologous to the transformed host cell, although it is to be understood that such control regions need not be derived from the genes native to the specific species chosen as a host.
Initiation control regions or promoters, which are useful to drive expression of the recombinant polypeptide in the desired microbial host cell are numerous and familiar to those skilled in the art. Virtually any promoter capable of driving these genes is suitable for the subject technology including but not limited to CYCI, HIS3, GALI, GALIO, ADHI, PGK, PH05, GAPDH, ADCI, TRPI, URA3, LEU2, ENO, TPI (useful for expression in Saccharomyces); AOXI (useful for expression in Pichia); and lac, trp, JPL, IPR, T7, tac, and trc (useful for expression in Escherichia coli).
Termination control regions may also be derived from various genes native to the microbial hosts. A termination site optionally may be included for the microbial hosts described herein.
In plant cells, the expression vectors of the subject technology can include a coding region operably linked to promoters capable of directing expression of the recombinant polypeptide of the subject technology in the desired tissues at the desired stage of development. For reasons of convenience, the polynucleotides to be expressed may comprise promoter sequences and translation leader sequences derived from the same polynucleotide. 3′ non-coding sequences encoding transcription termination signals should also be present. The expression vectors may also comprise one or more introns to facilitate polynucleotide expression.
For plant host cells, any combination of any promoter and any terminator capable of inducing expression of a coding region may be used in the vector sequences of the subject technology. Some suitable examples of promoters and terminators include those from nopaline synthase (nos), octopine synthase (ocs) and cauliflower mosaic virus (CaMV) genes. One type of efficient plant promoter that may be used is a high-level plant promoter. Such promoters, in operable linkage with an expression vector of the subject technology should be capable of promoting the expression of the vector. High level plant promoters that may be used in the subject technology include the promoter of the small subunit (ss) of the ribulose-1,5-bisphosphate carboxylase for example from soybean (Berry-Lowe et al., J. M
Precursor Synthesis to Reb J or Reb I
As previously stated steviol glycosides are the chemical compounds responsible for the sweet taste of the leaves of the South American plant Stevia rebaudiana (Asteraceae) and in the plant Rubus chingii (Rosaceae). These compounds are glycosylated diterpenes. Specifically, their molecules can be viewed as a steviol molecule, with its hydroxyl hydrogen atom replaced by a glucose molecule to form an ester, and a hydroxyl hydrogen with combinations of glucose and rhamnose to form an acetal.
One method of making the compounds of interest in the current disclosure is to take common or inexpensive precursors such as steviol, stevioside, Reb A or rubososide derived chemically or produced via biosynthesis in engineered microbes such as bacteria and/or yeast and to synthesize targeted steviol glycosides through known or inexpensive methods, such as Reb J or Reb I.
Aspects of the present disclosure relate to methods involving recombinantly expressing enzymes in a microbial system capable of producing steviol. In general, such enzymes may include: a copalyl diphosphate synthase (CPS), a kaurene synthase (KS) and a geranylgeranyl diphosphate to synthase (GGPPS) enzyme. This should occur in a microbial strain that expresses an endogenous isoprenoid synthesis pathway, such as the non-mevalonate (MEP) pathway or the mevalonic acid pathway (MVA). In some embodiments, the cell is a bacterial cell, including E. coli, or yeast cell such as a Saccharomyces cell, Pichia cell, or a Yarrowia cell. In some embodiments, the cell is an algal cell or a plant cell.
Thereafter, the precursor is recovered from the fermentation culture for use in chemical synthesis. Typically, this is steviol though it can be kaurene, or a steviol glycoside from the cell culture. In some embodiments, the steviol, kaurene and/or steviol glycosides is recovered from the gas phase while in other embodiments, an organic layer or polymeric resin is added to the cell culture, and the kaurene, steviol and/or steviol glycosides is recovered from the organic layer or polymeric resin. In some embodiments, the steviol glycoside is selected from rebaudioside A, rebaudioside B, rebaudioside C, rebaudioside N, rebaudioside E, rebaudioside F, rebaudioside J or dulcoside A. In some embodiments, the terpenoid produced is steviobioside or stevioside. It should also be appreciated that in some embodiments, at least one enzymatic step, such as one or more glycosylation steps, are performed ex vivo.
Part of the disclosure is the production of the Reb J steviol glycoside, which can be subject to further enzymatic conversion to Reb N. In some embodiments, Reb A can be converted to Reb J using a rhamnosyltransferase (RhaT) as described herein, e.g., EU11, EUCP1, HV1, UGT2E-B, or NX114, and a rhamnose donor moiety such as UDP-rhamnose. In some embodiments, the biosynthesis for the conversion of microbially produced steviol to a desired steviol glycoside (e.g., Reb N) occurs when the diterpenoid steviol is converted to rubusoside and stevioside using multi-step chemical assembly of sugar moiety into the steviol backbone utilizing specifically identified and/or modified enzymes created by the inventors. In addition to the EU11, EUCP1, HV1 and UGT76G1 enzymes utilized herein other enzymes were identified that can work to deliver these steviol rebaudiosides as well. For example, it was determined that other UGT enzymes (CP1 and CP2 respectively—amino acid sequences at SEQ ID NO: 11 and SEQ ID NO: 13) and the UGT76G1-AtSUS1 fusion enzyme can convert Reb J to Reb N.
Part of the disclosure is the production of the Reb I steviol glycoside from Reb A, which can be subject to further enzymatic conversion to Reb N. In some embodiments, the biosynthesis for the conversion of Reb A to Reb I occurs by reacting Reb A with a glucose donor moiety in the presence of a recombinant polypeptide having glucosyltranserase activity. In some embodiments, the glucose donor moiety is generated in situ. In some embodiments, the glucose donor moiety is added to the reaction. In some embodiments, the recombinant polypeptide having glucosyltranserase activity further comprises sucrose synthase activity. For example, in some embodiments, an enzyme identified as UGT76G1 (SEQ ID NO: 7) can convert Reb A to Reb I. In some embodiments, other UGT enzymes (e.g., CP1 and CP2 respectively—amino acid sequences at SEQ ID NO: 11 and SEQ ID NO: 13) can convert Reb A to Reb I. In some embodiments, the UCT76G1-AtSUS1 fusion enzyme can convert Reb A to Reb I. In some embodiments, Reb I can be converted to Reb N using a rhamnosyltransferase (RhaT) as described herein, e.g., EU11, EUCP1, HV1, UGT2E-B, or NX114, and a rhamnose donor moiety such as UDP-rhamnose.
Biosynthesis of Steviol Glycosides
As described herein, the recombinant polypeptides of the present technology have UDP-glycosyltransferase (UDP-gluocosytransferase and/or UDP-rhamnosyltransferase) activities and are useful for developing biosynthetic methods for preparing steviol glycosides that are either not present in nature or typically of low abundance in natural sources, such as rebaudioside J, rebaudioside I and rebaudioside N, respectively. The recombinant polypeptides of the present technology have UDP-glycosyltransferase activities, are useful for developing biosynthetic methods for preparing steviol glycosides, such as rebaudioside J or rebaudioside I, and reaching the synthetic production of rebaudioside N.
The substrate or starting steviol glycoside composition, can be any natural or synthetic compound capable of being converted into a steviol glycoside compound in a reaction catalyzed by one or more UDP-rhamnosyltransferases. For example, the substrate can be natural stevia extract, steviol, steviol-13-O-glucoside, steviol-19-O-glucoside, steviol-1,2-bioside, rubusoside, stevioside, rebaudioside A, rebaudioside B or rebaudioside I. The substrate can be a pure compound or a mixture of different compounds. Preferably, the substrate includes a compound selected from the group consisting of rubusoside, stevioside, steviol, rebaudioside A, rebaudioside B, rebaudioside J and combinations thereof.
The method described herein also provides a coupling reaction system in which the recombinant peptides described herein can function in combination with one or more additional enzymes to improve the efficiency or modify the outcome of the overall biosynthesis of steviol glycoside compounds. For example, the additional enzymes may regenerate the UDP-rhamnose needed for the rhamnosylation reaction (see e.g., Pei et al., “Construction of a novel UDP-rhamnose regeneration system by a two-enzyme reaction system and application in glycosylation of flavonoid,” Biochemical Engineering Journal, 139: 33-42 (2018), the entire disclosure of which is incorporated herein by reference) and the additional enzyme may regenerate the UDP-glucose needed for the glycosylation reaction by converting the UDP produced from the glycosylation reaction back to UDP-glucose (using, for example, sucrose as a donor of the glucose residue), thus improving the efficiency of the glycosylation reaction.
In another embodiment, the method of the subject technology further includes incubating a recombinant and novel UDP-rhamnosyltransferase (RhaT) according to the current disclosure with the substrate and one or more additional recombinant polypeptides (e.g., a recombinant UGT) described herein. The recombinant UGT can catalyze a different glycosylation reaction than the one catalyzed by the recombinant polypeptide of the subject technology leading to the production of Reb J and Reb N.
Suitable UDP-glycosyltransferase includes any UGT known in the art as capable of catalyzing one or more reactions in the biosynthesis of steviol glycoside compounds, such as UGT85C2, UGT74G1, UGT76G1, or the functional homologs thereof.
In some embodiments, in the in vitro method of the subject technology, UDP-glucose and/or UDP-L-rhamnose can be included in the buffer at a concentration of from about 0.2 mM to about 5 mM, preferably from about 0.5 mM to about 2 mM, more preferably from about 0.7 mM to about 1.5 mM. In an embodiment, when a recombinant sucrose synthase is included in the reaction, sucrose is also included in the buffer at a concentration of from about 100 mM to about 500 mM, preferably from about 200 mM to about 400 mM, more preferably from about 250 mM to about 350 mM.
In some embodiments, in the in vitro method of the subject technology, the weight ratio of the recombinant polypeptide to the substrate, on a dry weight basis, is from about 1:100 to about 1:5, preferably from about 1:50 to about 1:10, more preferably from about 1:25 to about 1:15.
In some embodiments, the reaction temperature of the in vitro method is from about 20° C. to about 40° C., suitably from 25° C. to about 37° C.
One skilled in the art will recognize that the steviol glycoside composition produced by the method described herein can be further purified and mixed with other steviol glycosides, flavors, or sweeteners to obtain a desired flavor or sweetener composition. For example, a composition enriched with Reb J or Reb N produced as described herein can be mixed with a natural stevia extract containing rebaudioside A as the predominant steviol glycoside, or with other synthetic or natural steviol glycoside products to make a desired sweetener composition. Alternatively, a substantially purified steviol glycoside (e.g., rebaudioside J and rebaudioside N) obtained from the methods described herein can be combined with other sweeteners, such as sucrose, maltodextrin, aspartame, sucralose, neotame, acesulfame potassium, and saccharin. The amount of steviol glycoside relative to other sweeteners can be adjusted to obtain a desired taste, as known in the art. The steviol glycoside described herein (including rebaudioside N, rebaudioside I, rebaudioside J, or a combination thereof) can be included in food products (such as beverages, soft drinks, ice cream, dairy products, confectioneries, cereals, chewing gum, baked goods, etc.), dietary supplements, medical nutrition, as well as pharmaceutical products.
Analysis of Sequence Similarity Using Identity Scoring
As used herein “sequence identity” refers to the extent to which two optimally aligned polynucleotide or peptide sequences are invariant throughout a window of alignment of components, e.g., nucleotides or amino acids. An “identity fraction” for aligned segments of a test sequence and a reference sequence is the number of identical components which are shared by the two aligned sequences divided by the total number of components in reference sequence segment, i.e., the entire reference sequence or a smaller defined part of the reference sequence.
As used herein, the term “percent sequence identity” or “percent identity” refers to the percentage of identical nucleotides in a linear polynucleotide sequence of a reference (“query”) polynucleotide molecule (or its complementary strand) as compared to a test (“subject”) polynucleotide molecule (or its complementary strand) when the two sequences are optimally aligned (with appropriate nucleotide insertions, deletions, or gaps totaling less than 20 percent of the reference sequence over the window of comparison). Optimal alignment of sequences for aligning a comparison window are well known to those skilled in the art and may be conducted by tools such as the local homology algorithm of Smith and Waterman, the homology alignment algorithm of Needleman and Wunsch, the search for similarity method of Pearson and Lipman, and preferably by computerized implementations of these algorithms such as GAP, BESTFIT, FASTA, and TFASTA available as part of the GCG® Wisconsin Package® (Accelrys Inc., Burlington, Mass.). An “identity fraction” for aligned segments of a test sequence and a reference sequence is the number of identical components which are shared by the two aligned sequences divided by the total number of components in the reference sequence segment, i.e., the entire reference sequence or a smaller defined part of the reference sequence. Percent sequence identity is represented as the identity fraction multiplied by 100. The comparison of one or more polynucleotide sequences may be to a full-length polynucleotide sequence or a portion thereof, or to a longer polynucleotide sequence. For purposes of this disclosure “percent identity” may also be determined using BLASTX version 2.0 for translated nucleotide sequences and BLASTN version 2.0 for polynucleotide sequences.
The percent of sequence identity is preferably determined using the “Best Fit” or “Gap” program of the Sequence Analysis Software Package™ (Version 10; Genetics Computer Group, Inc., Madison, Wis.). “Gap” utilizes the algorithm of Needleman and Wunsch (Needleman and Wunsch, J
Useful methods for determining sequence identity are also disclosed in the Basic Local Alignment Search Tool (BLAST) programs which are publicly available from National Center Biotechnology Information (NCBI) at the National Library of Medicine, National Institute of Health, Bethesda, Md. 20894; see BLAST Manual, Altschul et al., NCBI, NLM, NIH; Altschul et al., J. M
As used herein, the term “substantial percent sequence identity” refers to a percent sequence identity of at least about 70% sequence identity, at least about 80% sequence identity, at least about 85% identity, at least about 90% sequence identity, or even greater sequence identity, such as about 98% or about 99% sequence identity. Thus, one embodiment of the disclosure is a polynucleotide molecule that has at least about 70% sequence identity, at least about 80% sequence identity, at least about 85% identity, at least about 90% sequence identity, or even greater sequence identity, such as about 98% or about 99% sequence identity with a polynucleotide sequence described herein. Polynucleotide molecules that have the activity of the 1,2 RhaT and UGT enzymes of the current disclosure are capable of directing the production of a variety of steviol glycosides and have a substantial percent sequence identity to the polynucleotide sequences provided herein and are encompassed within the scope of this disclosure.
Identity and Similarity
Identity is the fraction of amino acids that are the same between a pair of sequences after an alignment of the sequences (which can be done using only sequence information or structural information or some other information, but usually it is based on sequence information alone), and similarity is the score assigned based on an alignment using some similarity matrix. The similarity index can be any one of the following BLOSUM62, PAM250, or GONNET, or any matrix used by one skilled in the art for the sequence alignment of proteins.
Identity is the degree of correspondence between two sub-sequences (no gaps between the sequences). An identity of 25% or higher implies similarity of function, while 18-25% implies similarity of structure or function. Keep in mind that two completely unrelated or random sequences (that are greater than 100 residues) can have higher than 20% identity. Similarity is the degree of resemblance between two sequences when they are compared. This is dependent on their identity.
As is evident from the foregoing description, certain aspects of the present disclosure are not limited by the particular details of the examples illustrated herein, and it is therefore contemplated that other modifications and applications, or equivalents thereof, will occur to those skilled in the art. It is accordingly intended that the claims shall cover all such modifications and applications that do not depart from the spirit and scope of the present disclosure.
Moreover, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure belongs. Although any methods and materials similar to or equivalent to or those described herein can be used in the practice or testing of the present disclosure, the preferred methods and materials are described above.
Although the foregoing invention has been described in some detail by way of illustration and example for purposes of understanding, it will be apparent to those skilled in the art that certain changes and modifications may be practiced. Therefore, the description and examples should not be construed as limiting the scope of the invention, which is delineated by the appended claims.
EXAMPLES Example 1: Enzymatic Activity Screening of 1,2 RhaT EnzymesPhylogenetic, gene cluster, and protein BLAST analyses were used to identify candidate 1,2 RhaT transferase genes. Full-length DNA fragments of candidate 1,2 RhaT transferase genes were optimized and synthesized according to the codon preference of E. coli (Genscript, NJ). The synthesized DNA fragments were cloned into a bacterial expression vector pETite N-His SUMO Kan Vector (Lucigen).
Each expression construct was transformed into E. coli BL21 (DE3), which was subsequently grown in LB media containing 50 μg/mL kanamycin at 37° C. until reaching an OD600 of 0.8-1.0. Protein expression was induced by adding 1 mM of isopropyl β-D-1-thiogalactopyranoside (IPTG), and the culture was incubated further at 16° C. for 22 hours. Cells were harvested by centrifugation (3,000×g; 10 min; 4° C.). The cell pellets were collected and were either used immediately or stored at −80° C.
The cell pellets typically were re-suspended in lysis buffer (50 mM potassium phosphate buffer, pH 7.2, 25 μg/ml lysozyme, 5 μg/ml DNase I, 20 mM imidazole, 500 mM NaCl, 10% glycerol, and 0.4% Triton X-100). The cells were disrupted by sonication at 4° C., and the cell debris was clarified by centrifugation (18,000×g; 30 min). The supernatant was loaded to an equilibrated (equilibration buffer: 50 mM potassium phosphate buffer, pH 7.2, 20 mM imidazole, 500 mM NaCl, 10% glycerol) Ni-NTA (Qiagen) affinity column. After loading of the protein samples, the column was washed with equilibration buffer to remove unbound contaminant proteins. The His-tagged 1,2 RhaT recombinant polypeptides were eluted with an equilibration buffer containing 250 mM of imidazole.
The purified candidate 1,2 RhaT recombinant polypeptides were assayed for UDP-rhamnosyltransferase activity. Typically, the recombinant polypeptide (20-50 μg) was tested in a 200 μl in vitro reaction system. The reaction system contains 50 mM of potassium phosphate buffer, pH 7.2, 3 mM of MgCl2, 0.25-1 mg/ml of rebaudioside A and UDP-L-rhamnose. The reaction was performed at 30-37° C. and terminated by adding 1-butanol.
The product samples were extracted three times with 200 μL of 1-butanol. The pooled fraction was dried and dissolved in 100 μL of 80% methanol for high-performance liquid chromatography (HPLC) analysis.
HPLC analysis was performed using a Dionex UPLC ultimate 3000 system (Sunnyvale, Calif.), including a quaternary pump, a temperature controlled column compartment, an auto sampler and a UV absorbance detector. A Synergi Hydro-RP column (Phenomenex) with guard column was used for the characterization of steviol glycosides in the pooled samples. The detection wavelength used in the HPLC analysis was 210 nm.
After activity screening, several 1,2 RhaT enzymes having UDP-rhamnosyltransferase activity were identified for bioconversion of Reb A to Reb J.
Example 2: Enzymatic Bioconversion of Reb A to Reb JThe biosynthesis of rebaudioside J involves glucosylation and rhamnosylation of the aglycone steviol. Specifically, Reb J can be produced by rhamnosylation of the C-2′ of the 19-O-glucose of Reb A, i.e., via a 1,2 rhamnosylation (
It has been reported that EU11, EUCP1 and HV1 have glucosylation activity and are capable of transferring a glucose moiety from UDP-glucose to steviol glycosides, producing compounds such as stevioside, rebaudioside KA, rebaudioside E, rebaudioside D, rebaudioside V, and rebaudioside D3 etc.
As shown in
Collectively, these data demonstrate that each of the enzymes EU11, EUCP1, HV1, UGT2E-B, and NX114 exhibits 1,2 RhaT activity and is capable of transferring a rhamnose moiety from a UDP-L-rhamnose donor to the C-2′ of the 19-O-glucose moiety of Reb A in an a 1,2 rhamnosylation reaction, thereby producing Reb J.
Example 3: Confirmation of Reb J Production by LC-MS AnalysisTo confirm the identity of the compounds produced in the reactions catalyzed by the 1,2 RhaT candidate enzymes, LC-MS analyses were performed using a Synergy Hydro-RP column. Mobile phase A was 0.1% formic acid in water, and mobile phase B was 0.1% formic acid in acetonitrile. The flow rate was 0.6 ml/min. Mass spectrometry analysis of the samples was done on the Q Exactive Hybrid Quadrupole-Orbitrap Mass Spectrometer (Thermo Fisher Scientific) with an optimized method in positive ion mode.
LC-MS analysis confirmed that the products obtained from the rhamsylation reaction catalyzed by EU11, EUCP1, HV1, UGT2E-B, and NX114, respectively, have the same mass [(M+Na+) 1135.48 m/z] and retention time (2.61 min) as the Reb J standard.
Example 4: Enzymatic Bioconversion of Reb J to Reb NReferring again to
To identify suitable UGTs for bioconversion of Reb J to Reb N, enzyme candidates were assayed using the Reb J products obtained from Example 1.
After terminating Reb J production by heating (to denature the 1,2 RhaT enzymes), UGT candidate enzymes (5 μg of enzymes per 200 μl of reaction mixture) and 1 mM of UDP-glucose was added into the reaction mixture. The glucosylation reaction was performed at 37° C. for 3 hours.
Based on the structure of UGT76G1, the inventors were able to design CP1 (SEQ ID NO: 11) which is a circular permutation of UGT76G1, CP2 (SEQ ID NO: 13) which is a mutant of CP1 and was generated by inserting a linker between the C-terminal and N-terminal of CP1, and a fusion enzyme GS (SEQ ID NO: 15) that includes a UDP-glycosyltransferase (UGT) domain, UGT76G1, coupled to a sucrose synthase (SUS) domain, AtSUS1.
Referring to
To evaluate further the activity of a UGT-SUS fusion enzyme in the bioconversion of Reb J to Reb N, assays were performed in the presence of UDP and sucrose but without UDP-glucose using the following enzymatic systems: (1) UGT76G1 (SEQ ID NO: 7) by itself, (2) addition of both a UDP-glycosyltransferase UGT76G1 (SEQ ID NO: 7) and a sucrose synthase AtSUS1 (SEQ ID NO: 17), and (3) fusion enzyme GS (SEQ ID NO: 15) including UDP-glycosyltransferase (UGT) domain, UGT76G1, coupled to a sucrose synthase (SUS) domain, AtSUS1, which has both UDP-glycosyltransferase activity and sucrose synthase activity.
As shown in
To confirm further the identity of the compounds produced in the reactions catalyzed by the UGT enzymes, LC-MS analyses were performed using a Synergy Hydro-RP column. Mobile phase A was 0.1% formic acid in water, and mobile phase B was 0.1% formic acid in acetonitrile. The flow rate was 0.6 ml/min. Mass spectrometry analysis of the samples was done on the Q Exactive Hybrid Quadrupole-Orbitrap Mass Spectrometer (Thermo Fisher Scientific) with an optimized method in positive ion mode.
LC-MS analysis confirmed that the products obtained from the glucosylation reaction catalyzed by UGT76G1, CP1, CP2, and GS, respectively, using Reb J (produced from the rhamnosylation reaction catalyzed by EU11/EUCP1/HV1/UGT2E-B/NX114 as described in Example 1) as the substrate, have the same mass [(M+Na+) 1297.53 m/z] and retention time (2.43 min) as the Reb N standard.
Sequences of Interest:
Sequences:
Claims
1. A biosynthetic method of preparing rebaudioside N, the method comprising:
- reacting a steviol glycoside composition comprising rebaudioside A with a rhamnose donor moiety in the presence of a first recombinant polypeptide having 1,2-rhamnosytransferase activity to produce rebaudioside J; wherein said first recombinant polypeptide comprises the amino acid sequence of SEQ ID NO: 3; and
- reacting said rebaudioside J with a glucose donor moiety in the presence of a second recombinant polypeptide having glucosyltransferase activity to produce rebaudioside N, wherein said second recombinant polypeptide comprises the amino acid sequence of SEQ ID NO: 13.
2. The method of claim 1, comprising expressing said first recombinant polypeptide in a transformed cell.
3. The method of claim 2, wherein the transformed cell is selected from the group consisting of a yeast, a non-steviol glycoside producing plant, an alga, a fungus, and a bacterium.
4. The method of claim 2, wherein the reacting step is performed in the transformed cell.
5. The method of claim 2, comprising isolating said first recombinant polypeptide from the transformed cell and the reacting step is performed in vitro.
6. The method of claim 1, wherein the rhamnose donor moiety is UDP-L-rhamnose.
7. The method of claim 1, comprising reacting said rebaudioside J with the glucose donor moiety in the presence of a third recombinant polypeptide having sucrose synthase activity.
8. The method of claim 1, further comprising isolating rebaudioside N from the steviol glycoside composition.
9. The method of claim 1, further comprising purifying rebaudioside N to obtain a steviol glycoside composition enriched with rebaudioside N.
10. A biosynthetic method of preparing rebaudioside J, the method comprising:
- reacting a steviol glycoside composition comprising rebaudioside A with a rhamnose donor moiety in the presence of a recombinant polypeptide having 1,2-rhamnosytransferase activity; wherein said recombinant polypeptide comprises the amino acid sequence of SEQ ID NO: 3.
11. The method of claim 10, comprising expressing said recombinant polypeptide in a transformed cell.
12. The method of claim 11, wherein the transformed cell is selected from the group consisting of a yeast, a non-steviol glycoside producing plant, an alga, a fungus, and a bacterium.
13. The method of claim 11, wherein the reacting step is performed in the transformed cell.
14. The method of claim 11, comprising isolating said recombinant polypeptide from the transformed cell and the reacting step is performed in vitro.
15. The method of claim 10, wherein the rhamnose donor moiety is UDP-L-rhamnose.
16. The method of claim 10, further comprising isolating rebaudioside J from the steviol glycoside composition.
17. The method of claim 10, further comprising purifying rebaudioside J to obtain a steviol glycoside composition enriched with rebaudioside J.
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Type: Grant
Filed: Nov 8, 2019
Date of Patent: Jan 5, 2021
Patent Publication Number: 20200087695
Assignee: Conagen Inc. (Bedford, MA)
Inventors: Guohong Mao (Burlington, MA), Michael Batten (Westford, MA), Phillip Hunt (Tewksbury, MA), Oliver (Xiaodan) Yu (Lexington, MA)
Primary Examiner: Delia M Ramirez
Application Number: 16/679,032
